The chemical diffusion coefficient and oxygen-transfer coefficients of selected compositions in the series were studied using the conductivity relaxation technique. Measurements were performed in the temperature range 600-850Â°C and oxygen partial pressure to 1 bar. Chemical diffusivity and oxygen surface transfer in the perovskites appear to be highly correlated. The general trend displayed is that both parameters decrease with decreasing pO2 below about bar at all temperatures. This is attributed to ordering of induced vacancies at low oxygen partial pressures. The observation that the correlation between both parameters extends even to the lowest values in this study suggests a key role of the concentration of mobile oxygen vacancies, rather than of the extent of oxygen nonstoichiometry, in determining the rates of both processes. The characteristic thickness , which equals the ratio of the chemical diffusion coefficient to the surface transfer coefficient, shows only a weak dependence on oxygen partial pressure and temperature. For different compositions , is found to vary between 50 and 150 µm.